Chapter 5- Overview of Nanomaterials - Nanoyou€¦ · · 2012-07-18Self-healing adhesives ... Polymers ... Biomimetic nanomaterials The first class of engineered nanomaterials

NANOYOU Teachers Training Kit in Nanoscience and Nanotechnologies

Chapter 5 – Overview of Nanomaterials Module 1- Fundamental concepts in nanoscience and nanotechnologies

Written by Luisa Filipponi and Duncan Sutherland Interdisciplinary Nanoscience Centre (iNANO) Aarhus University, Denmark September 2010 Creative Commons Attribution ShareAlike 3.0 unless indicated in text or figure captions. This document has been created in the context of the NANOYOU project (WP4). All information is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. The document reflects solely the views of its authors. The European Commission is not liable for any use that may be made of the information contained therein.

NANOYOU Teachers Training Kit – Module 1– Chapter 5

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The research leading to these results has received funding from the European Community’s Seventh Framework Programme

(FP7/2007-2013) under grant agreement n° 233433

Contents

Biomimetic nanomaterials ............................................................................................................................................. 5

Gecko-inspired adhesive (or Bio-rubber) ................................................................................................................... 5

Self-healing adhesives ................................................................................................................................................ 6

Biomimetic membranes, capsules and bioreactors ................................................................................................... 6

Biomimetic energy nanomaterials ............................................................................................................................. 7

Self-assembled nanomaterials ....................................................................................................................................... 7

Liquid crystals ............................................................................................................................................................ 8

Liquid crystal self-assembly ................................................................................................................................... 9

Liquid crystals´ phases and their properties ........................................................................................................ 10

LCs applications ................................................................................................................................................... 11

Nanostructured Metals and Alloys .............................................................................................................................. 12

Metal nanoparticles ................................................................................................................................................. 12

Plasmonic structures ............................................................................................................................................ 13

Reinforcements .................................................................................................................................................... 14

Environmental applications ................................................................................................................................. 14

Nanocrystalline metals ............................................................................................................................................ 15

Ferrofluids ................................................................................................................................................................ 15

Polymers ...................................................................................................................................................................... 16

Conductive Polymers ................................................................................................................................................ 16


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Block Copolymers ..................................................................................................................................................... 16

Responsive “smart” polymers .............................................................................................................................. 18

Biomedical applications ....................................................................................................................................... 18

Nano-reactors ...................................................................................................................................................... 19

Artificial moving parts ......................................................................................................................................... 19

Other applications ............................................................................................................................................... 19

Polymeric nanofibres ............................................................................................................................................... 19

Semiconductors ........................................................................................................................................................... 21

Quantum dots .......................................................................................................................................................... 21

Semiconducting oxides ............................................................................................................................................ 22

Titanium dioxide .................................................................................................................................................. 22

Zinc dioxide .......................................................................................................................................................... 24

ITO ....................................................................................................................................................................... 25

Photonic crystals ...................................................................................................................................................... 25

Ceramic and glassy materials ...................................................................................................................................... 26

Porous alumina ........................................................................................................................................................ 26

Zeolites ..................................................................................................................................................................... 26

Aerogels ................................................................................................................................................................... 27

Carbon-based materials ............................................................................................................................................... 27

Carbon nanotubes .................................................................................................................................................... 29

Mechanical properties ......................................................................................................................................... 30

Electrical and thermal properties ........................................................................................................................ 31


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Chemical reactivity .............................................................................................................................................. 32

Applications ......................................................................................................................................................... 32

Composites .................................................................................................................................................................. 33

Inorganic nano composites ...................................................................................................................................... 34

Structural nanocomposites .................................................................................................................................. 34

Nanocomposites with enhanced magnetic properties ........................................................................................ 36

Polymer Nanocomposites ........................................................................................................................................ 37

Nanoparticle- polymer nanocomposites .............................................................................................................. 39

Carbon nanotubes in polymer composites ........................................................................................................... 40

Polymer-clay nanocomposites ............................................................................................................................. 41

Nanocoatings ............................................................................................................................................................... 43

Tribological nanocoatings ........................................................................................................................................ 44

Responsive nanocoatings ......................................................................................................................................... 44

Self-cleaning surfaces .......................................................................................................................................... 46


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Chapter 5: Overview of Nanomaterials This chapter provides an overview of nanomaterials, their properties and functions. The application

areas of the cited nanomaterials are also mentioned. The second Module of this Teachers Training Kit is

devoted to applications of nanotechnologies.

Biomimetic nanomaterials

The first class of engineered nanomaterials that we review is Biomimetic materials. As mentioned in

Chapter 2 of Module 1 (“Natural Nanomaterials”), Nature is the best nanotechnology platform.

Through thousands of years of evolution Nature has developed an enormous array of materials, ranging

from feathers, to shells, wood, bone and many more that have intricate hierarchical structures at the

nano, micron and macro levels which confer specific properties on the material, such as strength,

lightweight, permeability, colour, etc. Natural materials provide an amazing platform inspiring material

engineers to fabricate advanced materials that possess specific functions. As a matter of fact numerous

“macro” materials we use today were developed after inspiration from natural materials. One example

is Velcro, which was developed in 1948 by a Swiss engineer named George de Mistral who was inspired

by the mechanism that enables cockleburs to cling to dog hair and fabric. Nanostructures in the natural

material often play a crucial role, which inspires scientists to mimic them starting from a molecular level

(molecular biomimetics). This is also called biomimetic nanotechnology. Here we provide some

examples of these biomimetic nanomaterials.

Gecko-inspired adhesive (or Bio-rubber)

In Chapter 2 we discussed the adhesive properties of the gecko foot, and how these are not related to a

glue in the foot, but rather to van der Waals and capillary forces exerted by millions of nanostructures

(called setae) that make up the foot. This allows the animal to walk upside down, against gravity, on

many different surfaces, even on wet ones. Moreover a gecko can walk on a dirty surface without losing

adherence, since its feet are also self-cleaning. A truly amazing material! Scientists have been inspired

by this animal to design and fabricate adhesives for numerous applications. For instance a group of

researchers at the University of California, Berkeley, has developed adhesive gecko-foot-like surfaces for

use in climbing robots. The adhesive is made of patches of microfiber arrays with 42 million

polypropylene microfibers per cm2. The patches can support up to 9 N cm-2: a 2 cm2 patch can support a

load of 400 g. This result is very close to the loads supported by a gecko, which are about 10 N cm-2. This


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gecko-like adhesive is very similar in functionality to the natural gecko foot, but yet not as good.

Researchers still have to make it topography-independent (capable of attaching to any surface) and self-

cleaning. In another university a biomimetic gecko tape has been produced using polymer surfaces

covered with carbon nanotube hairs, which can stick and un-stick on a variety of surfaces, including

Teflon.

Other gecko-inspired nanomaterials are under development for medical applications, which

will be covered in Chapter 1 of Module 2 (“Applications of Nanotechnologies: Medicine &

Healthcare”).

Self-healing adhesives

Diatoms are a type of algae with nanostructured amorphous silica surfaces. Some diatom species have

evolved strong self-healing underwater adhesives. Some are free-floating; others have adhesive

properties in water: for instance diatoms in the Antarctic seas can attach to ice. Others secrete viscous

mucilage which binds colonies together while protecting the silica shells from wear as they rub against

each other. Molluscs are another species of animals that have adhesive properties under water. Both

diatoms and molluscs have strong underwater glues that can also resist stress and self-heal if necessary.

For these reasons they serve as a biomimetic model for self-healing materials. Researchers have studied

these natural adhesives and found that their self-healing properties are due to the properties of the

proteins that compose them. These proteins have “sacrificial” bonds that allow the molecule to be

reversibly stretched by their breaking and re-bonding. This sacrificial bond behaviour has been observed

in many other materials, such as wool. The detailed analysis of these natural materials is inspiring new

nano-adhesives with self-healing properties.

Biomimetic membranes, capsules and bioreactors

The bilipid membrane has served as a biomimetic model for decades. A simple example is liposomes

(lipid vesicles) which are easily formed by shaking oil vigorously in water. Planar supported bilayers are

also inspired by the lipid membrane and are formed by simply “dipping” a suitable substrate inside an

organic aqueous phase.

Inside the cell there are numerous self-assembled structures that encapsulate specific

biomolecules and release them as a consequence of molecular signalling. These structures

are inspiring a wide range of nanocapsules for application in drug delivery. In some cases

bacteria or viruses are used as templates for fabricating these nanocapsules. These


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NANOYOU GAME Nature-inspired nanomaterials are used in one of the NANOYOU Jigsaw puzzle game. Students

need to match the nanomaterial, to its application, connecting it to a specific need. The game can be found at:

http://nanoyou.eu/en/play-nano.html

nanomaterials are described in more detail in Chapter 1 of Module 2 (“Applications of

Nanotechnologies: Medicine & Healthcare”).

Biomimetic energy nanomaterials

Many challenges that we are now facing in the energy area (improvements needed in solar panels,

hydrogen fuel cells, rechargeable batteries etc.) can be solved through the use of nano-engineered

materials. Some of these materials are developed through direct inspiration from Nature, such as the

new types of solar photovoltaic cells, which try to imitate the natural nanomachinery of photosynthesis.

Another interesting example is that of using battery electrodes with self-assembling nanostructures

grown by genetic engineered viruses.

Both examples will be described in more detail in Chapter 3 of Module 2 (“Applications of

Nanotechnologies: Energy”)

Self-assembled nanomaterials

The concept of self-assembly derives from observing that, in natural biological processes, molecules self-

assemble to create complex structures with nanoscale precision. Examples are the formation of the DNA

double helix or the formation of the membrane cell from phospholipids. In self-assembly, sub-units

spontaneously organise and aggregate into stable, well defined structures through non covalent

interaction. This process is guided by information that is coded into the characteristics of the sub-units

and the final structure is reached by equilibrating to the form of the lowest free energy. An external

factor, such as a change in temperature or a change in pH, can disrupt this organisation. For instance, a

protein self-assembles in a specific structure, but if exposed to conditions such as high heat or high

acidity, it can denature, which means that its structure is damaged, and the protein unfolds. This means

that the protein loses its function as its structure is damaged. So in Nature self-organised structures

have specific functions.


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Figure 2. Diagram showing where

the branch of "liquid crystals" fits

into the classification of physical

chemistry of condensed matter.

Figure 1. The liquid crystal phase is in between a solid and a

liquid phase (Image credit: L. Filipponi, iNANO, Aarhus

University, Creative Commons Attribution ShareAlike 3.0).

Molecules in Nature change conformation and move from one self-organised structure into another as

they bind to certain ions or atoms. Many examples could be given, like haemoglobulin (which captures

and releases an iron ion), or the potassium-sodium pump, chlorophyll, etc.

The use of self-assembly to create new materials is a bottom-up approach to nanofabrication, and is

thus an essential tool in nanotechnology (see Chapter 7 of Module 1 “Fabrication techniques”). Instead

of carving nanostructures out of larger materials (which is the typical top-down approach, like

micromachining and microlithography, used to fabricate integrated electronic circuits), nanostructures

are created from the bottom, from atomic building blocks that self-assemble in larger structures. In the

laboratory scientists can make use of this self-organisation of matter as a way of programming the

building of novel structures with specific functions.

Liquid crystals

A liquid crystal is a fourth state of matter: it has

properties between those of a conventional

liquid and those of a solid crystal. Like a liquid, it

flows, and like a crystal, it can display long-range

molecular order (Figure 1). In terms of

classifications, LCs (together with polymers and

colloids), are often classified as “soft matter”

(Figure 2) and treated under the branch of

physical chemistry of condensed matter.


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Figure 5. Example of the self-organisation of

anisometric (i.e., with asymmetrical parts)

molecules in liquid-crystalline phases. On the left:

rod-like molecules form a nematic liquid, in which

the longitudinal axes of the molecules are aligned

parallel to a common preferred direction

("director"). On the right: disc-like (discotic)

molecules arrange to molecule-stacks (columns), in

which the longitudinal axes are also aligned parallel

to the director. As a result of their orientational

order, liquid crystals exhibit anisotropic physical

properties, just like crystals. (Reprinted from:

http://www.ipc.uni-

stuttgart.de/~giesselm/AG_Giesselmann/Forschung

/Fluessigkristalle/Fluessigkristalle.html).

A fascinating and characteristic feature of liquid-crystalline systems is that they change their molecular

and supermolecular organisation drastically as an effect of very small external perturbations: The

molecules in liquid crystal displays for instance are reoriented by relatively weak electrical fields. If one

dissolves a small amount of chiral molecules in an achiral liquid-crystalline host phase, this results in

remarkable macroscopic chirality effects, ranging from helical superstructures to the appearance of

ferroelectricity.

Liquid crystal self-assembly

Liquid crystals are partly ordered materials, somewhere between their solid and liquid phases. This

means that LCs combine the fluidity of ordinary liquids with the interesting electrical and optical

properties of crystalline solids.

The molecules of liquid crystals are often shaped like rods or plates or some other forms that encourage

them to align collectively along a certain direction (Figure 5).


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Figure 3. Schematic representation of molecules in a solid (left,

molecules are well organised), in a liquid crystal (centre)

molecules have a long range distance order) and in a liquid (right)

molecules are not ordered. (Image credit: Copyright IPSE

Educational Resources, University of Wisconsin Madison)

Liquid crystals are temperature sensitive since they turn into solid if it is too cold and into liquid if it is

too hot. This phenomenon can, for instance, be observed on laptop screens when it is very hot or very

cold.

Liquid crystals´ phases and their properties

A liquid crystal is formed by the self-assembly of molecules in ordered structures, or phases. An external

perturbation, such as a change in temperature or magnetic field, even very small, can induce the LC to

assume a different phase. Different phases can be distinguished by their different optical properties

(Figure 3).

Liquid crystals are divided into three groups

- Thermotropic LC: they consist of organic molecules, typically having coupled double bonds, and exhibit

a phase transition as temperature is changed (Figure 4, left);

:

- Lyotropic LC: they consist of organic molecules, typically amphiphilic (water-loving) and exhibit a phase

transition as a function of both temperature and concentration of the LC molecules in a solvent

(typically water) (Figure 4, right);

- Metallotropic LC: they are composed of both organic and inorganic molecules, and their LC transition

depends not only on temperature and concentration, but also on the organic-inorganic composition

ratio.


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Figure 4. (Left) chemical structure of N-

(4-Methoxybenzylidene)-4-butylaniline

(MBBA); (Right): Structure of lyotropic

liquid crystal: 1 is a bilayer and 2 is a

micelle. The red heads of surfactant

molecules are in contact with water,

whereas the tails are immersed in oil

(blue). (Image credit: Wiki commons,

Creative Commons Attribution

ShareAlike 3.0).

Lyotropic liquid-crystalline phases are abundant in living

systems, such as biological membranes, cell membranes, many proteins (like the protein solution

extruded by a spider to generate silk), as well as tobacco mosaic virus. Soap is another well known

material which is in fact a lyotropic LC. The thickness of the soap bubbles determines the colours of light

they reflect.

LCs applications

The order of liquid crystals can be manipulated with mechanical, magnetic or electric forces. What is

interesting is that this change of order can be obtained with very small variations of these forces. The

properties of liquid crystals are useful in many applications. The colour of some liquid crystals depends

on the orientation of its molecules, so any perturbation that disturbs this orientation (e.g. a difference in

magnetic or electric field; a difference in temperature; or the presence of certain chemicals) can be

detected with a colour change.

EXPERIMENT B in the Experiment Module deals with synthesising and testing different

temperature sensitive liquid crystals. In this experiment students test how different LCs

are sensitive to different temperature ranges and build an LC thermometer.

The details of how a thermotropic LC works are described in the background document for

EXPERIMENT B in the Experiment Module.

Liquid crystals are routinely used in displays for cell phones, cameras, laptop computers and other

electronics. In these displays, an electric field changes the orientation of the molecules in the liquid

crystal, and affects the polarisation of light passing through them. Because of their sensitivity to


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Figure 6. Gold colloid is

ruby red, not golden!

(Image credit: L.

Filipponi, iNANO, Aarhus

University, Creative

Commons Attribution

ShareAlike 3.0).

temperature, and the property of changing colour, they are also used in thermometers. In miniaturised

sensors, liquid crystals can detect certain chemicals, electric fields and changes in temperature.

Liquid crystal displays (LCDs) are discussed in Chapter 4 of Module 2 “Applications of

Nanotechnologies: ICT”.

In the future LCs might have very interesting applications. Recently, it has been found that the inclusion

of specific molecules, often in the order of nanometres, in the liquid crystal can lead to new electrical

and optical properties in the LC. For instance, LC materials have been modified to induce new photonic

functions, for application to waveguide and optical storage (ICT application area). Another class of LCs

under development is photoconductive LCs, with the aim of having new materials for PVs cells and light

emitting diodes (Energy application area). Another area of study is to modify LCs in ways that allow

them to be used in stimuli-responsive materials and as templates for nanoporous polymers (potential

nanomedicine application area).

Nanostructured Metals and Alloys

This section covers metal nanoparticles – nanocrystalline materials consisting of a single metal or of

alloys made of two or more metals and nanostructured metal surfaces.

Metal nanoparticles

Metal nanoparticles are a clear example of how at the

nanometre scale the properties of matter can change.

For instance metal gold is notably yellow in colour and

used for jewellery. As the noblest of all metals, gold is

very stable (for instance, it does not react with oxygen

or sulphur). However, if gold is shrunk to a

nanoparticle, it changes colour, becoming red if it is spherical (Figure 6) and

even colourless if it is shaped in a ring. Moreover, gold nanoparticles become very reactive and can be

used as new catalysts (this will be discussed in Chapter 2 “Applications of nanotechnologies:

Environment” of Module 2).


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Figure 7. (Top) LSPR sensor based on a change of

refractive index induced by analyte attachment on a

functionalised surface; (Bottom) LSPR sensor based on a

change in nanoparticle aggregation induced by

attachment on functionalised nanoparticles.

Plasmonic structures

Noble metal nanoparticles, meaning gold, silver, platinum and palladium nanoparticles, show localised

surface plasmon-resonances (LSPR), an effect that was described in Chapter 4 of Module 2

(“Fundamental ‘nano-effects’”). The energy of the LSPR depends on the particle shape, size,

composition, inter-particle spacing and dielectric environment. The surface of the nanoparticles can be

functionalised with numerous chemical and biochemical molecules enabling specific binding of organic

molecules such as antibodies, making them useful in sensors .

For this reason they are of special interest for optical detection and sensing in analytical chemistry and

molecular biology. The refractive index can be used as the sensing parameter: changes in the local

dielectric environment, induced by the sensing process, are used to detect the binding of molecules in

the particle nano-environment. The change in aggregation between the nanoparticles as a result of

analyte attachment affects the LSPR energy, so this effect can be used for highly miniaturised sensors.

Figure 7 schematically illustrates these two LSPR-based detection methods.

Localised surface plasmons have been explored in a range of nanoparticle shapes such as disks,

triangles, spheres and stars. Also more complex structures have been studied, such as holes in thin

metal film, nanoshells and nanorings. Based on nanostructured metallic surfaces a variety of optical

applications are possible. An important feature of plasmonic structures is that they allow label-free

detection, which is important in optical sensing. Currently plasmonic components are being investigated

with respect to future applications in cancer therapy, solar cells, waveguides, optical interconnection,

camera LEDs, OLEDs and more.


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The application of metal nanoparticles in medical diagnostics and targeted treatment is

discussed in detail in Chapter 1 of Module 2 “Applications of Nanotechnologies: Medicine

and Healthcare”.

Reinforcements

Metal nanoparticles are used as reinforcements in alloys for application in lightweight construction

within the aerospace sector and increasingly in the automotive sector. The method is used for instance

to harden steel. For instance, titanium nanoparticles are used as alloy compound to steel, and the

resulting material shows improved properties with respect to robustness, ductility, corrosion and

temperature resistance. Particles of iron carbide are also precipitated into steel to make it harder. The

nanoparticles block the movement of the dislocations in the crystalline material (this effect was

described in details in Chapter 4 of Module 1 “Fundamental ‘nano-effects’”) increasing hardness. The

trade off between steel strength and ductility is an important issue, since modern construction requires

high strength whereas safety and stress redistribution require high ductility. The presence of hard

nanoparticles in the steel matrix could lead to a material with a combination of these properties,

effectively matching high strength with exceptional ductility.

Environmental applications

Some forms of metal nanoparticles have important environment applications.

Zero-valent (Fe0) iron nanoparticles are under investigation for the remediation of contaminated

groundwater and soil. Iron when exposed to air oxidises easily to rust; however, when it oxidises

around contaminants such as trichloroethylene (TCE), carbon tetrachloride, dioxins, or PCBs, these

organic molecules are broken down into simple, far less toxic carbon compounds. Iron nanoparticles

are more effective than conventional ‘iron powder’, which is already used to clean up industrial

wastes. Iron nanoparticles are 10 to 1000 times more reactive then commonly used iron powders.

Silver nanoparticles have a strong anti-bacterial capacity. They are used in numerous products to

prevent or reduce the adherence of bacteria to surfaces.

Metal nanoparticles and their use in environment applications are covered in the Chapter 2

of Module 2 “Applications of Nanotechnologies: Environment”.


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Figure 8 A ferrofluid with a

magnet underneath. (Image

credit: G. F. Maxwell, Wiki

Commons, GNU Free

documentation licence).

Nanocrystalline metals

Nanocrystalline metals are classical metals and alloys that have an ultra-fine crystalline structure below

100 nm. They exhibit extraordinary mechanical and physical properties which make them interesting

for many applications. Examples of nanocrystalline metal materials are Al, Mg, and Al-Mg alloys which

offer high strength and are lightweight. Other examples are Ti- and Ti-Al-alloys.

Some crystalline metals offer exceptional magnetic properties. An example is the “Finemet”

nanocrystalline soft magnetic alloys, which consist of melt-spun Fe-Si-B alloys containing small amounts

of Nb and Cu.

The application of nanomaterials in the field of magnetic materials is very important and promising, with

applications in magnetic recording, giant-magneto resistance, magnetic refrigeration and magnetic

sensing. These novel materials are composed of magnetic nanoparticles dispersed in a magnetic or

nonmagnetic matrix (particle-dispersed nanocomposites) or of stacked magnetic thin films (magnetic

multilayer nanocomposites).

We will cover the application of nanostructured magnetic materials for memory storage in

Chapter 4 of Module 2 “Applications of Nanotechnologies: ICT”.

Ferrofluids

Ferrofluids are colloidal mixtures composed of ferromagnetic or

ferrimagnetic nanoparticles (such as magnetite) suspended in a carrier

fluid, usually an organic solvent or water. The ferromagnetic nano-

particles are coated with a surfactant to prevent their agglomeration

(due to van der Waals and magnetic forces). Although the name may

suggest otherwise, ferrofluids do not display ferromagnetism, since they

do not retain magnetisation in the absence of an externally applied

field. Rather they display bulk-like paramagnetism, and are often

referred to as “superparamagnetic” materials due to their large

magnetic susceptibility. When a paramagnetic fluid is subjected to a

sufficiently strong vertical magnetic field, the surface spontaneously

forms a regular pattern of corrugations; this effect is known as the normal-field instability (Figure 8).


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Ferrofluids have numerous applications: in mechanical engineering, particularly for vehicle suspension

and breaking systems, due to their low-friction properties; in the ICT area, as liquid seals (ferrofluidic

seals) around the spinning drive shafts in hard disks; and in the biomedical sector as drug carriers.

Polymers

A polymer is a large molecule made of a chain of individual basic units called monomers joined together

in sequence. A copolymer is a macromolecule containing two or more types of monomers. When the

polymer is a good conductor of electricity, it is referred to as conductive polymer (or organic metal). In

this section nanostructuring of polymers and the effect this can have on polymers´ properties are

described. The focus is on co-polymers since these are extremely useful in nanotechnologies.

Conductive Polymers

Polymers that are good conductors of electricity are called conductive polymers and include

polyacetylene, polyaniline, polypyrrole, polythiophene and many more have been synthesised. These

polymers are characterised by having alternating double-single chemical bonds, so they are π-

conjugated. The π-conjugation of the carbon bonds along the oriented polymer chains provides a

pathway for the flow of conduction electrons, and is thus responsible for the good electrical conduction

properties of the material. A detailed SEM analysis of conductive polymers has revealed that these are

made of a sequence of metallic nanoparticles about 10nm in diameter. The high conductivity of

polymers such as polyacetylene and polyaniline is related to the nanostructure of the polymer.

Polyaniline and its analogues change colour when a suitable voltage is applied, or when reacting with

specific chemicals (electrochromic and chemochromic). For this reason they are promising for use in

light-emitting diodes (LEDs). Other applications are the surface finish of printed-circuit boards, corrosion

protection of metal surfaces, semitransparent antistatic coatings for electronic products, polymeric

batteries and electromagnetic shielding.

Block Copolymers

A copolymer is a macromolecule containing two or more type of monomers, and a block copolymer has

these basic units or monomer types joined together in long individual sequences called blocks. An

example is the diblock polymer (A)m(B)n, which is made of a linear sequence of m monomers of type A

joined together to a linear sequence of n monomers of type B. A transition section joints the two blocks:

[end group]-[polyA]m-[transition member]-[polyB]n-[end group]


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Figure 9. Different geometries formed by block copolymers in

selective solvent conditions. Image adapted from Smart et al.

Nano Today (2008), 3 (3-4), 38-45, Copyright (2008) with

permission from Elsevier.

Often, block copolymers are made of a hydrophilic (water-attracted) block, and a hydrophobic (water-

repellent) block. In general, macromolecules having hydrophilic and hydrophobic regions, such as lipids,

self-assemble in ordered structures when in water: the hydrophobic region packs together, avoiding

the water molecules, leaving the hydrophilic molecules to the exterior of the structure. In the same way,

block copolymers made of hydrophilic and hydrophobic blocks when mixed in a selective solvent, like

water, can self-assemble into ordered architectures in the nanoscale regime.

The geometry and degree of order of these

structures depends on the concentration and

the volume ratio between insoluble and

soluble blocks. Depending on these

parameters, the block copolymer can form

spherical micelles (nanospheres), cylindrical

micelles and membranes. Both cylindrical and

spherical micelles consist of a nonsoluble

(hydrophobic) core surrounded by a soluble

corona. Membranes are made of two

monolayers of block copolymer aligned to

form a sandwich-like membrane: soluble

block-insoluble block-soluble block. Molecules

that at low concentration form spherical

aggregates will assemble into cylindrical and

eventually membrane-like structures as the concentration is increased (Figure 9).

Among spherical micelles, if the lengths of the projections formed by the hydrophilic corona are short

compared to the sphere diameter, the nanostructure is called a “hairy nanosphere”, whereas if the

sphere is small and the projections long, it is called a “star polymer” (Figure 10).


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Figure 10. Sketch of a star polymer and a hairy polymer.

(Image credit: L. Filipponi, iNANO, Aarhus University,

Creative Commons Attribution ShareAlike 3.0).

Responsive “smart” polymers

Block copolymers form nanostructures that are very sensitive to external fields. For instance moderate

electrical fields or shear stimulation can trigger macroscopic re-arrangements in specific directions. This

is a versatile property for making materials that respond and change on demand. The intrinsic

macromolecular nature of these copolymers leads to very slow and kinetically controlled phase

transitions. Thus metastable or intermediate phases have longer lifetimes, which is desirable in

applications that want to exploit the phase transition properties of these materials.

Biomedical applications

The ability of block copolymers to form nanoparticles and nanostructures in aqueous solutions makes

them particularly useful for biomedical applications, such as therapeutics delivery, tissue engineering

and medical imaging. In the field of therapeutic delivery, materials that can encapsulate and release

drugs are needed. Hydrogels are very useful for the controlled release of drugs and block copolymer

hydrogels are particularly advantageous for the possibility of conferring some stimuli-activated

properties, such as temperature-sensitivity. Block copolymers form nanostructures with both

hydrophilic and hydrophobic areas, so they can form vesicles that can encapsulate and carry both

hydrophobic and hydrophilic therapeutic agents. Micelles formed using block copolymers have a

hydrophilic corona that makes them more resistant to the interaction of proteins, in particular plasma

proteins; therefore these types of micelles exhibit long circulation times in vivo. Insoluble domains can

also be engineered to exploit the sensitivity of specific hydrophobic polymers to external stimuli such as

pH, oxidative species, temperature and hydrolytic degradation.

Block copolymers are also of interest for preparing scaffolds for tissue engineering. For instance very

long micelles that mimic the natural extracellular have recently been prepared exploiting the self-

assembly properties of a peptide copolymer.


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Nano-reactors

The ability to generate compartmentalised volumes at the nanometre level is one of the fundamental

mechanisms used by cells in synthesising biomolecules and performing the biochemical processes

necessary for their function. This motif is now reassembled using block copolymer micelles and vesicles

as nanoreactors. This approach has been employed for carrying enzymatic reactions in nanosized

compartments.

It has been shown that this approach can also be expanded into a nonaqueous solvent. For instance a

recent work reports block polymer nanostructures in ionic liquid and the ability of micelles to shuttle

from an aqueous solvent to an ionic liquid as a function of temperature. This opens the way to using

block copolymer technology in more sophisticated synthetic routes.

Artificial moving parts

Block copolymers structures can also be used to mimic the ability of biomolecules to convert chemical

energy into mechanical energy, for instance by using a pH sensitive block copolymer whose micelles

swell according to pH variation. Thus these materials are being investigated to create artificial muscles

or moving nanostructures.

Other applications

The application of block copolymers is not limited to the biomedical field. They can be used in

conjunction with other materials to form block copolymer nanocomposites. For instance star polymers

are used in industry to improve metal strength.

Finally, block copolymers can form nanoporous membranes for applications in filtering systems and fuel

cell technology.

Polymeric nanofibres

Nanostructured fibrous materials, or nanofibres, are an important class of nanomaterials, now readily

available thanks to recent developments in electrospinning and related fabrication technologies. In

contrast to conventional woven fabrics, they have the typical structure illustrated in Figure 11.


Page 20 of 46



Figure 11. A typical sample of electrospun polystyrene. (Image credit: T. Uyar,

Interdisciplinary Nanoscience Canter (iNANO), Aarhus University. Copyright

2008. Reprint not permitted).

Figure 12. Overview of polymer nanofibres

applications.

Nanofibres have some unique properties: they are highly porous,

i.e., they have a large interconnected void volume in the range of 50%

or even greater than 90% and possess a very high surface-to-volume

ratio. It is possible to increase the mechanical stability of nanofibrous

structures by annealing the fabric so to join together the crossing

points of those fibres.

These properties make nanofibrous scaffolds useful for many biomedical and industrial applications.

In addition, researchers have succeeded in making coaxial nanofibres composed on two different

polymers or composite coaxial fibres. Researchers are also trying to make aligned nanofibres. These

types of materials, particularly if made of conducting polymers, could have important applications for

electronic and medical devices.

Nanofibres have a broad spectrum of

applications as schematised in Figure 12.

The two areas that have so far received most attention are the medical field (e.g. wound

healing membranes) and filtration applications. These will be covered in Chapter 1

“Applications of nanotechnologies: Medicine and healthcare” and Chapter 2

“Applications of nanotechnologies: Environment” of Module 2 respectively). Nanofibres are also being

considered for composite material reinforcement. As nanofibres are a relatively recent type of

nanomaterial it is expected that in the future many more applications will be explored.


Page 21 of 46



Figure 13. Ten distinguishable emission colours of ZnS-capped CdSe

QDs excited with a near-UV lamp. From left to right (blue to red), the

emission maxima are located at 443, 473, 481, 500, 518, 543, 565,

587, 610, and 655 nm. (Reprinted by permission of Macmillan

Publishers from Macmillan Publishers Ltd: Nature Biotech. (2001) 19,

631-35, Copyright 2001).

Semiconductors

Semiconductors, unlike metals, have a band gap. The band gap is between the valence band and the

conduction band. In intrinsic semiconductors which possess no impurities (like boron, silicon,

germanium, indium) there are no electronic states in the band gap. The properties of semiconductors, in

particular the band gap, are manipulated by addition of dopants – impurities able to donate charge

carriers in the form of electrons (n-type) or holes (p-type).

As with metals, the reduction of the size of the semiconductor triggers the insurgence of novel physical

properties. The most evident example is that of quantum dots.

Quantum dots

Quantum dots are made of semiconductor materials such as CdSe, ZnSe, CdTe, about 10 nm in size. As in

the case of metal nanoparticles, electrons in quantum dots are localised in space.

A QD has a discrete quantised energy spectrum, so it can absorb a specific wavelength and emit a

monochromatic colour. Depending on their size, QDs emit different colours, as shown in Figure 13. The

reason for this was explained in Chapter 4 of Module 2 (“Fundamental ‘nano-effects’”): the width of

the bandgap is related to the size of the semiconductor; smaller sizes lead to a blue shift in the emission

spectra.

QDs, like metal nanoparticles. hold great promise for nano-enhanced imaging which will

bring progress to fields like environmental monitoring, medical diagnostics and treatment.

QD are also being investigated as novel light sources to improve LED technology and in

solar cell technology. These applications of QDs in the energy and ICT sectors will be

reviewed in Chapter 3 “Applications of Nanotechnologies: Energy” and Chapter 4 “Information and

Communication Technologies” of Module 2.


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Semiconducting oxides

Semiconducting oxides like TiO2

Titanium dioxide

and ZnO in bulk (macro) form are widely used in industry in many

products. When they are in a nanoscale form, they display interesting physical properties that allow the

designing of new materials and the improvement of old ones. Below is a short description of these

properties.

Titanium dioxide (TiO2) is a mineral mainly found in two forms, rutile and anatase. Titanium dioxide is

the most widely used white pigment because of its brightness (white colour) and very high refractive

index (n=2.4). It is used in paints, plastics, toothpastes, papers, inks, foods and medicines. In sunscreens

with a physical blocker, titanium dioxide is used both because of its high refractive index and its

resistance to discolouration under ultraviolet light. This is because TiO2 is a UV filter, it absorbs UV light.

TiO2, particularly in the anatase form, can be employed also as a photocatalyst under UV light. It

oxidises water to create hydroxyl (OH) radicals and it can also oxidise oxygen or organic materials

directly. For this reason TiO2

Titanium nanoparticles (30-50nm, often referred to as nano-TiO

is added to confer sterilising, deodorising and anti-fouling properties to

paints, cements, windows and tiles.

2) are at the centre of much attention

due to their optical and catalytic properties: they retain the ability to absorb UV light but light scattering

is dramatically reduced, so that TiO2 goes from appearing white to transparent (a detailed explanation

of this effect is given in Chapter 4 of Module 1 (“Fundamental ‘nano-effects’”). Nano-TiO2 is thus

suitable for transparent coatings, and for new generation sunscreens, which are characterised by a high

protective factor but transparent appearance. The catalytic properties of TiO2 when in a nano-size are

also greatly enhanced by the large surface-to-volume ratio. This property is increasingly used for

chemical catalysis applications such as photo-catalytic purification of water and air to decompose

organic pollutants (solar photocatalytic remediation). Thin films of TiO2 are used in windows to confer

self-cleaning properties on the glass (This application of nano-TiO2 nanoparticles is reviewed in the

“Nanocoatings” section of this document).


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ELSA & SATETY TOPIC: The use of nanoparticles in sunscreen is under much attention because some

environmental organization, like Friends of the Earth Australia, have accused that the nanoparticles used in

these products can pass the skin barrier and enter the blood stream, causing unwanted harm. To support this

statement they refer to research published on the toxicity of nano-titanium and nano-zinc oxide. These studies

though often use amounts and exposure conditions (e.g., injection or inhalation) which are very different from

what consumers of these sunscreens would be exposed to (cream rubbed on skin, amount and frequency of

use, skin washed after some time, sun exposure, etc.). Manufactures claim that the nanoparticles used in the

sunscreen formula are agglomerated; therefore they have dimensions that don’t allow for skin-penetration.

Reseach seem to support this statement. Opponents reply that long term studies must be performed before

this can be demonstrated, and that skin types vary from person to person, not to mention cases of damaged

skin, so generalizations are not possible yet (studies are mainly done in vitro using healthy skin). Overall, more

research is called upon by all stakeholders, including regulators, to establish the safety of these products.

NANOYOU DILEMMA The example titanium dioxide in sunscreens is used in one of the NT Virtual Dialogue

debates (see www.nanoyou.eu/en/decide). In this dilemma students consider the case of sunscreens with

nanoparticles and reflect on their advantages and possible disadvantages.

Another important application of nano-TiO2 is in organic solar cells, like the Grätzel solar

cell, where nano-TiO2

One limitation of using TiO

is used as the catalyst to promote light-induced electrical charge

separation. The use of titanium dioxide in this type of solar cells is covered in more detail in

Chapter 2 “Applications of Nanotechnologies: Environment” of Module 2.

2 as a photocatalyst is that this material only absorbs UV light, which

represents about 5% of the solar spectrum. In this context, nanotechnology could bring an improvement

in the form of nanoparticles with surfaces modified with organic or inorganic dyes to expand the

photoresponse window of TiO2

33

from UV to visible light.


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Figure 14. Patterned ZnO grown as vertical pillars in

an arrayed format. (Reprinted with permission from

Xu et al., Journal of American Chemical Society 2008,

130, 14958-14959. Copyright 2008 American

Chemical Society.

Zinc dioxide

Zinc oxide (ZnO) has some similar properties to TiO2, i.e., its nanoparticles scatter light so it can be used

for transparent UV filters, in creams or coatings. Like TiO2 it is used for solar photocatalytic remediation

but compared to TiO2

A peculiarity of ZnO is that it has a tendency to grow in self-organised nanostructures. By controlling

crystal growth conditions, a variety of crystal shapes are possible. Researchers have been able to grow

nanoscale wires, rods, rings etc. ZnO nanocolumns are of particular interest since low-temperature

photoluminescence measurements have revealed intense and detailed ultraviolet light emission near

the optical band gap of ZnO at 3.37 eV. ZnO thus can act as an optical amplification medium and as a

laser resonator.

it has a weaker photocatalytic effect. ZnO also suffers from the same limitation of

absorbing only a fraction of the solar spectrum so research is underway to increase its photoresponse.

ZnO wires arrayed on a surface are also being investigated as piezoelectric elements for miniaturised

power sources. This would allow creating flexible, portable power sources that could be included in

textiles so that they are able to scavenge energy from body motion, light wind, air flow etc .

The use of ZnO for the development of miniaturised power sources is described in detail in

Chapter 3 of Module 2 “Applications of Nanotechnologies: Energy”.


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ITO

ITO is a semiconducting material whose main feature is the combination of electrical conductivity and

optical transparency. ITO is made of around 90% indium(III)-oxide (In2O3) and around 10% tin(V)-oxide

(SnO2

Photonic crystals

). It is widely used in its thin-film form as transparent electrodes in liquid crystal displays, touch

screens, LEDs, thin-film solar cells, semiconducting sensors, etc. ITO is an infrared absorber so it is now

used as a thermal insulation coating on window glass. Its anti-static properties make it additionally

useful in applications like packaging and storage of electronic equipment. Since the material is very

expensive, alternative materials, such as fluor tin oxides and aluminium zinc oxides are being

considered. The use of ITO in “smart coatings” is further discussed in the “Nanocoatings” section of this

document.

A photonic crystal consists of a lattice of periodic dielectric or metal-dielectric nanostructures that

affects the propagation of electromagnetic waves. Essentially, photonic crystals contain regularly

repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate

through this structure – or not – depending on their wavelength. The periodicity of the photonic crystal

structure has to be of the same length-scale as half the wavelength of the electromagnetic waves, i.e.

~200 nm (blue) to 350 nm (red) for photonic crystals operating in the visible part of the spectrum. Such

crystals have to be artificially fabricated by methods such as electron-beam lithography and X-ray

lithography.

Photonic crystals exist in Nature. For instance in Chapter 2 of Module 1 “Natural Nanomaterials” it was

shown how the beautiful blue wings of some butterflies owe their colour to their internal nanostructure,

which is in fact a photonic crystal structure.

Photonic crystals are now receiving much attention because of their potential in particular

in the optical-communication industry. Researchers are considering using light and

photonic crystals (in alternative to electrons travelling in wires) for the new generation

integrated circuits. Light can travel much faster in a dielectric medium than an electron in a wire, and it

can carry a larger amount of information per second. Given the impact that semiconductor materials

have had on every sector of society, photonic crystals could play an even greater role in the 21st

century. Photonics and their impact in the ICT sector are analysed in Chapter 4 of Module 2

(“Application of Nanotechnologies: ICT”).


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Figure 15. A schematic

representation of a zeolite. (Image

credit: Wikimedia Commons,

image for public domain).

Ceramic and glassy materials

Ceramic materials by definition are ionically bound; they are hard materials, both electrically and

thermally very stable. In this category are included for instance Al2O3, Si3N4, MgO, SiO2, Na2O, CaO, and

ZrO2

Ceramics exist both in a crystalline and non-crystalline form (for instance the broad family of “glass”).

Nanostructures within the material have important consequences for their properties, as discussed in

the next sections.

. Ceramics are characterised by being hard yet brittle, therefore in many cases they are used as

composites where they are mixed with other materials (e.g. metals) to increase their mechanical

performance. Composite hard nanocoatings are particularly important for cutting tools (see the section

“Inorganic composites” in this document).

Porous alumina

Porous alumina membranes produced by anodic aluminium oxidation are characterised by having

hexagonally close-packed channels with diameters ranging from 10nm to 250 nm or greater. This

material is often used as a template for the synthesis of other materials.

Zeolites

Zeolites are natural crystalline materials with pores having regular arrangements. They are also often

used in the template synthesis of nanomaterial. They can also be used to prepare organised structures

of a certain material to confer new optical properties on it. For instance, selenium

can be incorporated into the channels of

mordenite, a natural zeolite. The difference

between a mordenite-selenium crystal and a

natural selenium crystal is noteworthy: the

optical absorption spectrum is considerably

shifted to higher energies for the mordenite-selenium crystal.


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Figure 16. The thermal insulating

properties of a silica aerogel. (Image credit:

Wikimedia Commons, image for public

domain).

Aerogels

Silicon dioxide (SiO2

Due to their high porosity aerogels have an extremely high surface area and very low thermal

conductivity. Thus they are suitable for thermal insulation and as filter materials (Figure 16). Another

prominent property is their low specific weight, making them interesting for lightweight construction.

Aerogels are also interesting for their optical properties, namely high optical transparencies. Silica

aerogels are made of pores of about 10 nm arranged in distances between 10 and 100nm. They are

resistant and chemically inert to liquid metals, heat resistant up to 1200°C and non-toxic, thus they have

also biomedical applications, such as substrates for cell growth and analysis. One of the problems of

SiO

) is the main component of quartz. It is chemically robust

and finds widespread applications. In commercial products it appears as an

additive in rubber products for vehicle tyres but it is also the component of new

types of aerogels. Generally speaking an aerogel is a solid with up to 95% of the

volume consisting of nanoscale pores.

Aerogels are manufactured with a sol-

gel technique and can be made of

carbon, metal oxide, polymers or

silicates.

2

Carbon-based materials

aerogels that needs to be addressed is that this material has to be expensively protected against

humidity, since it is not water resistant and suffers from a loss of stability and thermal conductivity once

it gets wet.

In Nature there are some pure materials that have striking different properties even though they are

made of the same atoms. For instance, graphite and diamond (Figure 17): two very popular materials,

one used conventionally in pencils and the other in jewellery. These two materials could not be more

different: graphite is soft, light, flexible, and conducts electricity while diamond is extremely strong, hard

and does not conduct electricity. Both materials are made of atoms of carbon linked through strong

bindings (covalent), but in graphite each carbon atom uses three out of its four electrons to form single

bonds with its neighbours, forming a linear sheet, whereas in diamonds each carbon atom uses all its

four electrons to form four single bonds, resulting in a 3-D structure. The different properties of graphite


Page 28 of 46



Figure 18. Eight allotropes of carbon: (a) diamond, (b)

graphite, (c) lonsdaleite, (d) C60, (e) C540, (f) C70, (g)

amorphous carbon and (h) a carbon nanotube. Image by

Michael Ströc, (GNU Free Documentation License).

Figure 17. Two allotropes of carbon and their respective chemical

structure: (left, top and bottom): diamond; right (top and bottom):

graphite.(Image credit: Wiki Commons, Creative Commons

Attribution ShareAlike 3.0)

and diamond are a consequence of the different way the carbon atoms in the materials are bonded

together.

Graphite and diamond are two pure forms (allotropes) of

carbon. In 1985 a new allotrope of carbon was discovered

formed of 60 atoms of carbons linked together through

single covalent bonds arranged in a highly symmetrical,

closed shell that resembles a football. This material was

officially named Buckminsterfullerene and is often referred to as buckyball, fullerene or simply C60

In the early 1990s, an incredible new carbon form was discovered, carbon nanotubes. These appear like

graphite sheets rolled up with fullerene-type end caps, but have totally different properties compared to

graphite. Figure 18 shows different forms of carbon allotropes (image d and h are structures of a C

. Since

its discovery, fullerenes with 70, 80 and even more carbon atoms have been discovered.

60

It is now known that fullerenes and carbon

nanotubes form naturally in common places like

flames (produced by burning methane, ethylene

and benzene) and in soot. Scientists have now

developed methods to synthesise these

and

a nanotube, respectively).


Page 29 of 46



nanomaterials with control over their final properties.

Carbon nanotubes

Carbon nanotubes can appear as single-wall nanotubes (SWNTs), with a diameter of approximately

1.4nm, or multi-wall nanotubes (MWNTs), consisting of 2 to 30 concentric tubes with an outer diameter

of 30-50nm. The structure of an SWNT can be conceptualised by wrapping a one-atom-thick layer of

graphite called graphene into a cylinder. To complete the nanotube, imagine adding two half fullerenes

on each end of the nanotube.

Carbon nanotubes can range in length from a few tens of nanometres to several micrometers, and can

have metallic properties (comparable to or even better than copper) or can be semiconductors (like

silicon in transistors), depending on their structure. These nanomaterials are truly amazing and have

great potential in numerous fields as illustrated in Figure 19.


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Figure 19. Overview of application areas of carbon nanotubes. (Reprinted from: M.A.H. Hyder, Master Thesis,

Technical University of Hamburg-Harburg, Germany, Copyright M. A. H. Hyder 2003).

The application of CNTs in the energy, environment, medicine and ICT sectors are covered in

the respective application chapters in Module 2.

Mechanical properties

The chemical bonding of nanotubes is composed entirely of sp2 bonds (carbon double bonds) similar to

those of graphite (whereas in diamonds all bonds are sp3). This bonding structure, which is stronger than

the sp3

Theoretical predictions say that carbon nanotubes are 100 times stronger than steel but have

one sixth of its weight. Therefore they are ideal in lightweight construction, for instance for the

automotive and aviation industries. Carbon nanotubes are already used in some consumer

products to add strength (without compromising weight) such as tennis rackets.

bonds found in diamonds, provides the molecules with their unique strength. Nanotubes

naturally align themselves into “ropes” held together by Van der Waals forces.

The mechanical properties of carbon nanotubes are summarised in the table below. Young’s modulus is

a measure of how stiff, or elastic, a material is. The higher the value is, the less a material deforms when

a force is applied. Tensile strength describes the maximum force that can be applied per unit area

before the material snaps or breaks. A third interesting measure of a material is its density, which gives

an idea of how light a material is. From the table below it can be seen that wood is very light but weak,

while nanotubes are many times stronger than steel and yet much lighter.

Material Young´s

Modulus (GPa)

Tensile

Strength (GPa)

Density

(g/cm3)

Single-wall

nanotube

800 >30 1.8

Multi-wall

nanotube

800 >30 2.6


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Diamond 1140 >20 3.52

Graphite 8 0.2 2.25

Steel 208 0.4 7.8

Wood 16 0.008 0.6

Table 1

Because of their mechanical properties carbon nanotubes are very interesting as fillers in

polymeric and inorganic composites.

Electrical and thermal properties

The electrical properties of a material are based on the movement of electrons and the spaces or

“holes” they leave behind. These properties are based on the chemical and physical structure of the

material. In nanoscale materials some interesting electrical properties appear. Carbon nanotubes are

the best example of this effect at the nanoscale. If one considers “building” a nanotube by rolling up a

graphene sheet, the resulting nanotube can be conductive (and indeed, very conductive!) or

semiconductive with relatively large band gaps. The electrical properties of the nanotube depend on the

way it was “rolled up” (technically known as chirality). If it is rolled up so that its hexagons line up

straight along the tube’s axis, the nanotube acts as a metal (conductive). If it is rolled up on the diagonal,

so the hexagons spiral along the axis, it acts as a semiconductor.

Why is this so? Graphene (that is, one layer of graphite) is not an insulator, but also neither a metal nor

a semiconductor; it has electrical properties somehow in between, it is called a “semimetal”. When

rolled up, it leads to a structure that is either metallic or semiconducting. On the other hand diamond

has a tetrahedral structure (derived from the fact that carbons are hybridised sp3 rather than sp2

One interesting property found in single-walled carbon nanotubes (SWCNT) is that electric conductance

within them is ballistic (which means that all electrons that go into one end of the conductor come out

of the other end without scattering, regardless of how far they need to travel).

as in

graphene) and is an insulator.

Researchers are also investigating whether nanotubes can be superconductors near room temperature,

meaning ballistic conductors that also exhibit a resistance of zero. A superconductor can transport an

enormous amount of current flow at tiny voltages. At present known superconductors work at very low


Page 32 of 46



ELSA & SATETY TOPIC: Carbon nanotubes are an extraordinary material which could improve numerous current

materials, particularly some plastic composites, and be used in electronics and sensing. The range of application

is fairly broad so the question of their safety is important. Carbon nanotubes are formed as nanometre thin,

long fibres, so the memory quickly goes back to asbestos, their claimed exceptional properties, but also their

late-found toxicity (they cause cancer). Some recent research does suggest that carbon nanotubes, inhaled in

large amounts, can be significantly toxic. As with other nanomaterials, though, the question of safety is more

complex: carbon nanotubes are used as composites, embedded with polymers or other materials, so

consumers would not be directly exposed to them (with the exception of workers in industry). But what

happens when the product is disposed and the material degrades? Is waste containing carbon nanotubes safe?

Should this waste be treated differently and undergo special treatment? There is intense research to find

answers to these questions, but time is needed. Opponents advocate that until these answers are not found,

products containing carbon nanotubes should not be placed on the market, and workers should treat these as

hazard materials and wear the stringiest protection measures.

temperatures. This field of research is very important since if a material were superconductive at room

temperature, it would carry current with no resistance,

Nanotubes as superconductors and their applications are further discussed in Chapter 3

“Applications of Nanotechnologies: Energy” of Module 2.

with no energy lost as heat. This could lead to

faster, lower-power electronics and the ability to carry electricity long distances with 100 per cent

efficiency.

In terms of thermal properties, carbon nanotubes dissipate heat better than any other known material

and are excellent thermal conductors.

Chemical reactivity

Carbon nanotubes are very stable; they can withstand the attack of numerous chemicals and resist

exposure to a large temperature range. However, their chemical structure can be changed by the

addition of specific ligands with functional groups that allow interaction with different chemicals. This

allows using them in sensors.

Applications

Carbon nanotubes are very promising materials for numerous applications (these will be described in

more detail in Module 2). Applications include: nanomedicine (drug delivery), environment (chemical

sensors), energy (supercapacitors, hydrogen storage materials, solar cells), ICT (integrated circuits,

electronic paper), advanced materials for construction, transport, sports equipment and more.


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NANOYOU GAME Carbon nanotubes composites are used in one of the NANOYOU Jigsaw puzzle game. Students

need to match the nanomaterial, to its application, connecting it to a specific need. The game can be found at:

http://nanoyou.eu/en/play-nano.html

Scientists have now developed methods to control the synthesis of carbon nanotubes to obtain regular

structures with specific properties. To date, though, the synthetic processes lead to moderate amount of

CNTs and mostly very limited length (of the order of millimetres). Often these processes lead to CNTs

which are not totally pure (traces of the catalyst used in the synthesis are present in the product) and

this has been associated with toxicity issues of CNTs.

The cost associated with the production of carbon nanotubes is extremely high. In the future this cost

must be considerably reduced to allow large-scale production and use of CNTs.

Composites

The idea behind nanocomposites is to use building blocks with dimensions in the nanometre range to

design and create new materials with unprecedented flexibility and improvements in their physical

properties. This concept is exemplified in many naturally occurring materials, like bone, which is a

hierarchical nanocomposite built from ceramic tablets and organic binders (see Chapter 2 of Module 1

“Natural nanomaterials”). When designing the nanocomposite, scientists can choose constituents with

different structures and composition and hence properties, so that materials built from them can be

multifunctional.

As a general definition a nanocomposite is a conventional material reinforced by nanoscale particles or

nanostructures which are dispersed through the bulk material. The base material itself generally consists

of non-nanoscale matrices. Nanocomposite typically consists of an inorganic (host) solid containing an

organic component or vice versa or two (or more) inorganic/organic phases in some combinatorial form.

At least one component must be in the nanosize. In general, nanocomposite materials can demonstrate

different mechanical, electrical, optical, electrochemical, catalytic and structural properties which are

different from those of the individual components. Apart from the properties of the individual

components, interfaces in a nanocomposite play an important role in determining the overall properties

of the material. Due to the high surface area of nanostructures, nanocomposites present many

interfaces between the intermixed phases and often the special properties of the nanocomposite are a


Page 34 of 46



consequence of the interaction of its phases at interfaces. In comparison, the interfaces in conventional

composites constitute a much smaller volume fraction of the bulk material.

In this chapter, nanocomposites are sub-divided in two main groups: inorganic nanocomposites, which

are characterised by an inorganic matrix (e.g. ceramic) reinforced by nanoscale particles or

nanostructures of inorganic (e.g. metal) or organic (e.g. carbon-based) nature; and polymer

nanocomposites, which are characterised by an organic matrix (e.g. polymer) reinforced by nanoscale

particles or nanostructures by inorganic (e.g. clay) or organic nature.

Inorganic nano composites

High-performance ceramics are sought in many applications, like highly efficient gas turbines, aerospace

materials, automobiles etc. The field of ceramics that focuses on improving their mechanical properties

is referred to as structural ceramics.

Nanocomposite technology is also applicable to functional ceramics such as ferroelectric, piezoelectric,

varistor and ion-conducting materials. In this case the properties of these nanocomposites relate to the

dynamic behaviour of ionic and electronic species in electro-ceramic materials. Among these materials

here we limit the review to nanocomposite with enhanced magnetic properties.

Structural nanocomposites

Presently even the best processed ceramics pose many unsolved problems such as poor resistance to

creep, fatigue and thermal shock; degradation of mechanical properties at high temperatures; and low

fracture toughness and strength. To solve these problems one approach has been incorporating a

second phase such as particulates, platelets, whiskers, and fibres in the micron-size range at the matrix

grain boundaries. However, the results obtained with these methods have been generally disappointing.

Recently, the concept of nanocomposites have been considered, where nanometre-size second phase

dispersions are inserted into ceramic matrices. Large improvements in both the fracture toughness and

the strength of a ceramic can often be achieved with nanometre-range particles embedded in a matrix

of larger grains at their grain boundaries. These can involve the incorporation of a nano-ceramic in a

bulk ceramic, a nano-metal in a ceramic, or a nano-ceramic in a metal. Another possibility is the

incorporation of a polymer in a ceramic. Without going too much into the details, we list here some

examples of inorganic nanocomposites that have improved structural properties:


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The incorporation of fine SiC and Si3N4 nanoparticles in an alumina matrix (Al2O3

One possibility for the fabrication of advanced structural ceramics is the dispersion of metallic

second-phase particles into ceramics which improves their mechanical properties, such as

fracture toughness, and influences other properties including magnetic and optical properties.

Nanocomposites of this type are Al

, a structural

ceramic material) first demonstrated the concept of structural nanocomposites. The dispersion

of these particles have shown to improve the fracture toughness from 3 to 4.8 MPa m1/2 and

the strength from 350 to 1050 MPa at only 5 vol. % additions of SiC.

2O3/W or MgO/Fe. Granular films can also be made with a

ceramic phase embedded with nanosize metal granules (such as Fe/Al2O3, Fe/SiO2

Another possibility is to add fine, rigid ceramic reinforcements to a ductile metal or alloy

matrix (called metal matrix composites, or MMC). The reinforcement can be either in the form

of particles (e.g. silicon carbide, aluminium oxide), fibres (e.g. silicon oxide, carbon) or a mixture

of both (hybrid reinforcement). Materials produced with this method are particularly useful in

the aerospace, automotive and aircraft industry. The advantage of MMCs is that they combine

metallic properties (ductility and toughness) with ceramic characteristics (high strength and

modulus), leading to materials having greater strength to shear and compression and to higher

working temperature capabilities. The properties of MMCs are controlled by the size and

volume fraction of the reinforcements as well as by the nature of the matrix/reinforcement

interface. The attention is now oriented towards the incorporation of nanoparticles and

nanotubes for structural applications, since these materials exhibit even greater improvements

in the physical, mechanical and tribological properties compared to MMC with micron-size

reinforcements.

). Such films

often show unusual or enhanced transport, optical and magnetic properties. The inclusion of

nanosized metals in a thin ceramic film can transform it from an insulator to a conductive film.

Nanoscale ceramic powders with carbon nanotubes provide another method for creating dense

ceramic-matrix composites with enhanced mechanical properties. For instance hot pressed α-

alumina with mixed carbon nanotubes results in lightweight composites with enhanced

strength and fracture toughness compared to polycrystalline alumina. The processing

conditions greatly influence the properties of this material, though. In metal matrix composites

(MMC) the incorporation of carbon nanotubes is considered very promising since these


Page 36 of 46



materials have higher strength, stiffness and electrical conductivity compared to conventional

metals.

Nanocomposites with enhanced magnetic properties

Materials with outstanding magnetic properties are in high demand as these are employed in nearly all

important technical fields, e.g. electrical power, mechanical power, high-power electromotors,

miniature motors, computer elements, magnetic high-density recording, telecommunications,

navigation, aviation and space operations, medicine, sensor techniques, magnetic refrigeration,

materials testing and household applications. Recent developments in the field of magnetic materials

have involved materials with a nanocrystalline structure or, in the case of thin films, layers of nanometre

thickness.

Nanostructuring of bulk magnetic materials leads to soft or hard magnets with improved properties.

One example is the “Finemet” nanocrystalline soft magnetic alloys, which consist of melt-spun Fe-Si-B

alloys containing small amounts of Nb and Cu. When annealing at temperatures above the crystallisation

temperature, the Fe-Si-B-Nb-Cu amorphous phase transforms to a crystalline solid with grain size of

about 10nm. These alloys have excellent magnetic induction and large permeability, and a very small

coercive field.

Nanosized magnetic powders can have extreme properties and have no hysteresis at any temperature.

These materials are called superparamagnetic and one example is nanosized powders of a Ni-Fe-Co

alloy.

Nanostructuring has also been studied in the context of hard magnets (permanent magnets). The

strongest known magnets are made of neodymium (Nd), iron and boron (e.g. Nd2Fe14B). In these

materials is has been found that the coercive field decreases significantly below ~40 nm and the

remnant magnetisation increases. Another approach to improving the magnetisation curve of

permanent magnets has been to fabricate nanocomposites made of hard magnetic phases, such as

Nd2Fe14B and Sm2Fe17N3

The grain size of the material also influences the magnetisation saturation point. For instance the

magnetisation of ferrite increases significantly below a grain size of 20nm. Thus reducing the size of the

grains in the magnet increases the energy product (which is the product of magnetisation and

, within soft magnetic matrices (e.g. soft α-phase of iron). The effect of the

inclusion of a soft iron mixed in with a hard material is to increase the remnant field.


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Figure 20. Schematic representation of nanoscale

fillers. (Image credit: L. Filipponi, iNANO, Aarhus

University, Creative Commons Attribution ShareAlike

3.0).

coercivity). The coercivity also increases with decreasing the grain sizes. In the case of nanocomposite

magnetic films this is true if the grains are isolated (no interaction), but when the grains start contacting

and exchange interaction kicks in, the coercivity falls rapidly with grain size. The coercivity is thus highest

at percolation, when the grains just start touching each other. This effect is important in the context of

designing thin-film nanocomposites (magnetic multilayer nanocomposites), for instance for high

magnetic density recording.

Magnetic multilayer nanocomposites are becoming an essential component of the new

generation data storage devices like Magnetoresistive Random Access Memory (MRAM).

This and other nanostructured devices made of nanomagnets with applications in the ICT

sector are discussed in Chapter 4 of Module 2 (“Applications of Nanotechnologies: ICT”).

Polymer Nanocomposites

Polymer composites are materials where a polymer is filled with an inorganic synthetic and/or natural

compound in order to increase several properties, like heat resistance or mechanical strength, or to

decrease other properties, like electrical conductivity or permeability for gases like oxygen or water

vapour. Materials with synergistic properties are used to prepare composites with tailored

characteristics; for instance high-modulus but brittle carbon fibres are added to low-modulus polymers

to create a stiff, light-weight polymer composite with some degree of toughness. Current polymer

composites are really filled polymers, since these materials lack an intense interaction at the interface

between the two mixed partners. Progress in this field has involved moving from macro scale fillers to

micron-scale fillers, to even smaller fillers.

In recent years scientists have started to explore a new approach to the production of polymer

composites with the use of nanoscale fillers, in which the filler is below 100nm in at least one

dimension. Nanoscale fillers include: nanoparticles; nanotubes; and layered (also called “plate-like”)

inorganic materials such as clays (Figure 20).


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Although some nanofilled composites have been used for more than a century, such as carbon-black

and fumed-silica-filled polymers, only recently have researchers started to systematically produce and

study these materials. The motivation has been the realisation of the exceptional combined properties

that have been observed in some polymer nanocomposites. This, together with substantial development

in the chemical processing of nanoparticles and in the in situ processing of nanocomposites, has led to

unprecedented control over the morphology of these materials.

One of the most common reasons for adding fillers to polymers is to improve their mechanical

performance. In traditional composites this often compromises the ductility of the polymer and in some

cases negatively impacts its strength, because of stress concentration caused by the fillers. Well

dispersed nanofillers, such as nanoparticles or nanotubes, can improve the modulus and the strength

and maintain (or even improve) ductility because their small size does not create large stress

concentration. For all nanofillers, a key requirement is the homogeneous dispersion of the filler within

the polymer matrix. As will be discussed in the next sections, this is a challenge in many cases, and a

topic of intense research.

Although the scientific community has made remarkable progress in this field in the last years, polymer

nanocomposites have just started to be explored, and many research questions still need to be

addressed. What is clear so far is that the use of nanoscale fillers opens the way for the development of

materials with exceptional properties. For instance, nanoparticles do not scatter light significantly, thus

it is possible to make polymer composites with altered electrical or mechanical properties that remain

optically clear. Nanoparticles are also of interest not just for their small size, but for their inherent

unique properties. Carbon nanotubes for instance display the so far highest values of elastic modulus, as

high as 1 TPa, and strengths that can be as high as 500 GPa. This could allow for instance the fabrication

of polymeric composites with exceptional strength and flexibility.

Another outstanding property that the use of nanoscale fillers confers on nanocomposites is an

exceptionally large interfacial area. The increase of surface area below 100nm is dramatic. The interface

controls the degree of interaction between the filler and the polymer, and is thus responsible for the

composite properties. Thus the largest challenge in nanocomposite science is learning to control the

interface.

Nanoscale fillers can potentially allow the creation of a vast range of different polymeric materials with

advanced properties. In general, macroscopic reinforcement elements have the limitation of always


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containing imperfections, but as the reinforcements become smaller and smaller, structural perfection

could be reached. The ideal reinforcements would have atomic or molecular dimensions and be

intimately connected with the polymer. The use of nanoscale fillers also, however, introduces a series of

fabrication challenges. Because of their small size and high surface area, nanoscale fillers such as

nanoparticles have a strong tendency to agglomerate rather to disperse homogeneously in a matrix. This

leads to particle-matrix mixtures with high viscosities, which can make the processing of those materials

quite challenging. The result is that even the most exciting polymer nanocomposites have very low

fractions of particle content, and relatively weak mechanical properties when compared with those

predicted in theory. Therefore polymer nanocomposites are an exciting type of advanced materials that

hold great promise in many application, but which are still mainly in a development stage. The intense

research efforts in this area suggest however that these materials will become more readily accessible in

the near future.

Nanoparticle- polymer nanocomposites

Nanoparticles are a type of nanofillers that offer the opportunity of developing polymers with new or

advanced properties. As already discussed, the size of a nanoparticle affects its properties; for instance

Au nanoparticles have different optical absorption spectra depending on the particle size. One of the

advantages of using nanoparticles in polymer composites is that the particle size and distribution can be

tuned. Materials that cannot be grown easily as single crystals can be used at the nanoscale and

dispersed in a polymer to take advantage of the single-crystal properties.

In general, nanoparticle-filled polymers display better mechanical properties, at least at low volume

fractions and if well dispersed in the polymer. The reason is that nanoparticles are much smaller than

the critical crack size for polymers and thus do not initiate failure. Thus nanoparticles provide a way for

simultaneously toughening and strengthening polymers. Proper dispersion is critical, however, in

achieving this.

The size scale of nanoparticle/polymer composites ranges from hybrid nanocomposites, in which the

polymer matrix and the filler are so intimately mixed that they are no longer truly distinct, to discrete

particles in a continuous matrix. Hybrid nanocomposites often arise from the use of block copolymers,

and this is discussed in a separate section of this chapter (see the section “Polymers”).

In terms of properties, the use of nanoparticles in filling polymers can influence not only the polymer

mechanical properties, but also the polymer mobility and relaxation behaviour, which in turn are


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connected with the glass transition temperature of the polymer (Tg), that is, the temperature at which a

polymer becomes brittle on cooling or soft on heating. In general, adding well dispersed, exfoliated

nanofiller increases the Tg of the polymer. Nanoparticle-filled polymers also show an increase in the

abrasion resistance of the composite.

One of the most exciting prospects of using nanoparticles in polymer composites is to create composites

with combined functionalities, such as electrically conducting composites with good wear properties

that are optically clear. This is possible because nanoparticles do not scatter light.

Carbon nanotubes in polymer composites

Carbon nanotubes have very distinct properties compared to graphite, as summarised in Table 1 (page

29). In the context of nanocomposites, SWNT are the most promising nanotube fillers. Some properties

are particularly interesting, in particular their flexibility under mechanical stress, their behaviour under

high temperature conditions and their electrical properties. As with other applications that make use of

carbon nanotubes, in composites it has been observed that the processing conditions ultimately affect

nanotubes’ properties, and as a consequence the composite, as well as the nanotubes’ purity. Carbon

nanotubes can also be doped, for instance with nitrogen and boron, which changes their surface

reactivity. For instance, nitrogen atoms inserted into the lattice of nanotubes makes them more

dispersed in solution (carbon nanotubes are insoluble in water). Modified nanotubes present different

electrical and optical properties and hence their use could lead to composites with novel properties.

It should be noted that the inclusion of carbon nanotubes in a polymer does not necessarily improve

its mechanical properties. Although in theory the modulus of nanotubes is much higher than any

graphite fibre, and hence the composite should have outstanding mechanical properties, it has been

demonstrated that a number of variables influence this outcome. For SWNT composites, the SWNT are

in a bundle; until SWNT are isolated from the bundles or the bundles are crosslinked, the modulus of

composites made from these materials will be limited. For this reason researchers are concentrating on

developing processing methods that make it possible to obtain significant volume fractions of exfoliated

nanotubes. As will be discussed later for clay-polymer nanocomposites, the structural arrangement of

the nanofiller within the polymer is also important: if the nanotubes are not straight when placed in the

composite, the modulus of the composite significantly decreases.

Apart from mechanical enhancement of polymers, nanotubes are also of interest for their electrical

properties. Carbon nanotubes are inherently more conductive then graphite. It has been found for


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instance that nanotube/PPV composites show a large increase in electrical conductivity compared to

simple PPV, of nearly 8 orders of magnitude. Recently an improvement of 4.5 orders of magnitude in the

electrical conductivity of nanotube/PVA nanocomposites has been reported.

Finally, nanotube/polymer composites are promising in the context of light-emitting devices. These

devices were developed after the discovery of electroluminescence from conjugated polymer materials

(such as PPV). The practical advantages of polymer-based LEDs are low cost, low operating voltage, ease

of fabrication and flexibility. Small loadings of nanotubes in these polymer systems are used to tune the

colour of emitted light from organic LEDs.

Polymer-clay nanocomposites

In the late 1980s it was discovered that adding 5% in weight of nano-sized clays to Nylon 6, a synthetic

polymer, greatly increased its mechanical and thermal properties. Since then, polymer-clay

nanocomposites have been widely studied and many commercial products are available. These hybrid

materials are made of organic polymer matrices and clay fillers. Clays are a type of layered silicates that

are characterised by a fine 2D crystal structure; among these, mica has been the most studied. Mica is

made up of large sheets of silicate held together by relatively strong bonds. Smectic clays, such as

montmorillonite, have relatively weak bonds between layers. Each layer consists of two sheets of silica

held together by cations such as Li+, Na+, K+ and Ca2+.

For these layered silicates to be useful as fillers in nanocomposites, the layers must be separated, and

the clay mixed thoroughly in the polymer matrix. This is not trivial as silicate clays are inherently

hydrophilic, whereas polymers tend to be hydrophobic. The solution is to exchange the cations that

keep the layers in the silicate together with larger inorganic ions that can thus open the galleries

between the layers (intercalation). When the silicate layers are completely separated, the material is

called exfoliated. In the case of intercalation, extended polymer chains are present between clay layers,

resulting in a multilayer structure with alternating polymer/clay phases at repeated distances of few

nanometres; in the exfoliated state the silicate layers are totally separated and dispersed in a

continuous polymer matrix. Those two states are schematised in Figure 21.

The presence of the cations is necessary to

compensate for the overall negative charge of the single layers. The layers are 20-200 nm in diameter

laterally and come into aggregates called tactoids, which can be about 1 nm or more thick. Naturally

occurring clays include montmorillonite (MMT) and hecrite, and their synthetic equivalents are saponite

and laponite respectively.


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Figure 21. Principle of formation of clay monolayer

containing nanocomposites with enhanced mechanical

and barrier properties. (Reprinted with permission from:

Weiss, J.; Takhistov, P.; McClements, D. J., “Functional

Materials in Food Nanotechnology”. In Journal of Food

Science 2006; Vol. 71, pp R107-R116. Copyright 2006

Wiley-Blackwell).

As mentioned before, the fabrication of a polymer-clay nanocomposite requires mixing two

components that are intrinsically non-compatible. Surfactants are ionic, and thus interact well with the

clay, but not with the polymer. An ideal solution is the use of “macro-surfactants” like block copolymers

combining hydrophilic and hydrophobic blocks that can interact with the clay and the polymer

respectively. For instance poly(ethylene oxide), or PEO, is an excellent intercalation material.

Although homogeneous dispersion of the filler in the polymer is an important parameter, another

important aspect is the packing (or alignment) of the filler in the polymer. To understand this concept it

is useful to consider a natural nanocomposite, bone. The unique properties of bone are a list of

apparent contradictions: Rigid, but flexible; lightweight, but solid enough to support tissue; mechanically

strong, but porous. In order to meet these different demands, bone has a hierarchical structure that

extends from the nanoscale to the macroscopic length scale. The hierarchical structure of bone is

responsible for its load transfer ability (see Chapter 2 of Module 1 “Natural Nanomaterials” for details

on the structure of bone). Nanoparticle-filled polymer composites have mechanical properties that are

actually disappointing compared to theoretical predictions, and this is due to the difficulty in obtaining

well-dispersed large volume fractions of the reinforcing nanomaterial and a lack of structural control.

Taking this into consideration, some scientists have recently reported the fabrication of ultrastrong and

stiff layered polymer nanocomposites. In this work bottom-up assembly of clay/polymer nanocomposite

allowed the preparation of a homogeneous, transparent material, where the clay nanosheets have

planar orientation. It was found that the stiffness and tensile strength of these multilayer

nanocomposites are one order of magnitude greater than those of analogous nanocomposites.


Page 43 of 46



Figure 22. Schematic of possible structures for

nanostructured coatings. (Left) multilayered

microstructure (e.g., TiN/TiC) for nanocomposite

coatings; (right) homogeneous nanostructured

coating (e.g., NiCoCrAlY alloy). (Image credit: L.

Filipponi, iNANO, Aarhus University, Creative

Commons Attribution ShareAlike 3.0).

Nanoclays are also used as fillers in polymers to increase the thermal stability of polymers. This

property was first demonstrated in the late 1960s for montmorillonite/PMMA composites. The

dispersion of the clays is critical to increasing the thermal stability (that is, raising the degradation

temperature) of the polymer.

In addition to thermal stability, the flammability properties of many clay/polymer nanocomposites are

also improved. Combining traditional flame retardants with intercalated or, better, exfoliated clays can

result in further improvements in flame retardance.

Finally, polysaccharide/clay nanocomposites are a class of materials that are important especially for the

food industry. These composites make use of naturally occurring polymers, such as starch, mixed with

clay to make biopolymer film with enhanced properties, in particular permeability to water vapour.

Nanocoatings

Nanocoatings are a type of nanocomposites. The layer thickness of a nanocoating is usually in the 1-100

nm range. Nanocomposite films include multilayer thin-films, in which the phases are separated along

the thickness of the film, or granular films, in which the different phases are distributed within each

plane of the film (Figure 22).

Nanocoatings make it possible to change properties of some materials, for instance to change the

transmission of visible and IR radiation in glass, or to introduce new properties such as “self-cleaning”

effects. In this document this first type of nanocoating will be discussed under the umbrella term

“responsive nanocoatings”.

Another important area of application of nanocoatings is tribological coatings. Tribology is the science

and technology of interacting surfaces in relative motion. Tribological properties include friction,


Page 44 of 46



lubrication and wear. Tribological coatings are those coatings that are applied to the surface of a

component in order to control its friction and wear. In this area the term “thin films” is often employed

as an alternative to nanocoatings due to the fact that it is an area of innovation that started years ago

and has now reached the nanoscale.

Tribological nanocoatings

Tribological coatings play a key role in the performance of internal mechanical components of a vehicle,

such as the engine and powertrain. They are also key elements in cutting tools and machinery in

general. By reducing wear and friction these coatings increase the lifetime of the working material while

also reducing the dissipation of energy as heat, thus increasing the efficiency of the moving part. When

applied to machinery and tools, tribological coatings can reduce (or eliminate) the need for lubricants,

increase cutting speed, increase the rate of material removal, reduce maintenance costs or reduce

processing cycle times.

Traditional materials used in coatings for tribological applications are carbides, cemented carbides,

metal ceramic oxides, nitrides and carbon-based coatings. Since the microstructure controls many of the

physical properties of the coating, having a nano-scaled microstructure may lead to significant

improvements in the coatings´ mechanical properties (e.g. hardness), chemical properties (e.g. corrosion

resistance) and electrical properties. Thus nanocomposite coatings are now being investigated as

alternatives to the traditional approach of using specific alloying elements in single-phase coating

materials to improve, for instance, properties such as hardness. One type of nanocomposite coatings is

multilayered thin-films made of different layers in the order of nanometres. These films are mostly used

for their enhanced hardness and elastic moduli, which is higher in multilayered films than in

homogeneous thin films of either component, and for their wear properties. Commercial multilayer

coatings made with multilayer periods in the nanoscale range already exist, such as WC/C coatings used

in the cutting tool industry. Other examples are films of alternating layers of TiN and NbN, or TiAlN/CrN

multilayers, which are more efficient than TiAlN films.

Responsive nanocoatings

Responsive nanocoatings are those where the properties of the material in the coating react to

environmental conditions, such as light or heat, either in a passive or an active way. These coatings

allow changing the properties of some materials, such as glass, by conferring new or improved

properties.


Page 45 of 46



The use of glass is very common in modern buildings, since it allows the construction of transparent and

seemingly lightweight structures. However the relative high transmittance of visible and infrared (IR)

light is a major disadvantage, since this leads to a large heat transfer which is particularly undesirable in

summer. The problem is reversed in winter, when heat is dispersed through the glass. In order to

address these problems various types of nano-coatings that modulate light transmission in glass are

under investigation and being commercialised. The aim is to reduce indoor heating in summer, so less

air-conditioning is required to keep the atmosphere cool, with consequent energy saving. One type of

coating, referred to as “low-e” meaning “low emissivity”, is based on a thin silver film, about 10nm thick,

surrounded by dielectric layers. Metallic layers have been widely used to increase the reflectivity of light

(and reduce transmittance) for years, but they have the disadvantage of giving a mirror-like appearance.

Silver loses its metallic appearance when scaled to a nano-film, thus eliminating this problem. Such a

coating is commercialised by Von Ardenne.

“Low-e” coatings are a type of passive nano-coatings, since the properties of the layers are

unperturbed during its operation. Another class of coatings used in glasses are those often described as

dynamic or “smart coatings”. In this case the environmental conditions, such as radiation intensity or

temperature, induce a change in the properties of the coating (e.g. darkening of windows). When the

effect is a change in the colour (meaning also transparency) they are called “chromogenic smart

materials”. The change can be induced actively, by pressing a button. This is the case of electrochromic

coatings where applying a small voltage induces a change in transmittance, and in the case of

gaschromic coatings, which change their transmittance in the presence of specific gases. Gaschromic

glazing makes use of the properties of tungsten oxide thin films (WO3) which go from colourless to blue

in the presence of hydrogen with a suitable catalyst. Gaschromic windows follow a double pane model:

on one pane a film of WO3 is deposited with a thin layer of catalysts on top. Hydrogen gas is fed into the

gap producing colouration (windows colour with 1.1-10 % hydrogen, which is below the flammability

concentration). To switch the colour of the window back, another gas is purged in (oxygen). Smart

coatings can also be passive in the sense of changing their optical properties due to a change of external

temperature (thermochromic) or light incidence (photochromic). Another example of nanotechnology

applied to smart coatings is the use of a family of wavelength-selective films for manufacturing “heat

mirrors”. One of these materials is indium-tin-oxide (ITO), an infrared absorber. A 300 nm ITO coating on

glass provides more than 80% transmission for the wavelengths predominant in sunlight. The

transmission properties of the window can be varied by changing the thickness and material


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composition of the coating, so that a combination of materials could be used to produce smart windows

that reflect solar energy in summer but transmit solar energy in winter.

Self-cleaning surfaces

Another example of functional nanocoating is photocatalytic coatings (commercialised as “self-

cleaning” glass) which use the catalytic properties of titanium dioxide (TiO2). Pilkington Activ™ Self

Cleaning Glass is a commercial example of a glass with a photocatalytic coating that renders the material

easier to clean. Superhydrophobic coatings have surfaces that mimic the surface found in the lotus leaf

(see Chapter 2 “Natural Nanomaterials” for details) and are being developed for many applications that

require resistance to dirt and ease of cleaning. Both materials are covered in Chapter 2 of Module 2

“Application of nanotechnologies: Environment”.

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